Internal emails being treated as external emails

[JackBarlow224]

We have Exchange Online only -  We have an application that sends out emails to users within the company however we have found an issue where all our distribution group recipients aren't getting the emails due to this email being treated as "outside organization".

The email that's sending the mail is an internal Distribution group email which routes through an authorized connector.(I believe this is the cause of it being turned to "external" but unsure)

We cannot allow the distribution groups to receive outside org mail, is there a way for us to bypass the connector with all the emails that would be effected? (would need something more dynamic than the rules in admin center) or a way for the mail to not be converted to "outside Organization"?

[#2]

@JackBarlow224 I don't understand this bit "The email that's sending the mail is an internal Distribution group email which routes through an authorized connector"
DL's or groups by themselves can't send messages. Is the application sending the email to a DL? Is the application on-premises and using an EXO connector for message relaying?

[#3]

@Dan Snape 

The application sends the mail as this DL to other DL's and personal mailboxes (however the issue effects any application that sends emails to DL's)

The application is not on prem, we are fully cloud based but It is using an EXO connector.

However, it's since we added the EXO connector, these emails are being marked as "outside organization" but the IP's the mail is being sent from is authorized in the connector so I am at a bit of a loss on what's happened.

[#4]

I assume you're using Option 3 from the below:
https://learn.microsoft.com/en-gb/Exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-de...